Breast cancer is the most common form of cancer among US women in every major racial/ethnic group. In 2000, an estimated 26:4% of all cases were in women under age 50. Younger women are more likely to receive toxic multi-modal treatments that contribute to ovarian failure and early menopause, putting this growing group of women at risk for bone mineral loss, osteoporosis, and fractures. Reducing the risk is critical because young survivors have the greatest expected longevity. Aims: This study aims to test the hypotheses that young breast cancer survivors randomly assigned to an exercise intervention, compared to controls, will after one year 1) perform resistance and aerobic exercise more frequently and for longer periods of time, 2) experience less bone mineral loss as measured by spine DXA (dual energy X-ray absorptiometry) and biochemical markers of bone turnover, 3) be more likely to have a body mass index within recommended guidelines, 4) demonstrate a greater increase in lean body mass and a greater reduction in fat mass, and 5) report better mental and physical health. Methods: In a randomized intervention trial, 400 women who were age 50 or under at diagnosis with invasive breast cancer and have recently completed chemotherapy will be randomly assigned to the Y exercise program or the control group. All women will be assessed at pre- and post-test one year later. Each woman in the exercise group will receive a one-year YMCA membership and be assigned a Y Coach (personal trainer) who will assess her physical fitness, develop a tailored exercise program, monitor adherence, and provide social support and counseling. Women in the control group will receive a monthly health newsletter. Significance: This research will advance scientific knowledge about the effects of abrupt menopause due to adjuvant treatment on bone loss in young breast cancer survivors and the extent to which enrollment in an individualized YMCA exercise program emphasizing resistance training reduces bone loss in this population. The use of physiological and biochemical markers to measure bone turnover and partnering with the YMCA to provide individually tailored exercise are innovations with this group of women. Program dissemination will be facilitated by the stable nationwide infrastructure of community-based Y facilities.